Presently, photoactive semiconductor nanoparticles are potentially useful in constructing solar cells and for use as labels in various bioassays. Particles of titanium dioxide (TiO2) are attractive candidates for such applications. However, the band gap energy of undoped TiO2, which is ˜3.2 eV, prevents such undoped particles from being used in a number of applications. Accordingly, various schemes for reducing the band gap energy of such particles have been suggested. For example, in one application, particles of a titanium oxide are mixed with the monomer for conjugated polymer, then the monomer is polymerized with added oxidant. The composite particles of inorganic metal oxides and electroactive polymers have potential uses in photovoltaic cells. Unfortunately, the amount of polymer that is attached to the particles depends on a number of factors that are not easily controlled.
A second method for preparing coated nanoparticles is taught in Rajh, et al (J. Phys. Chem. B. 1999, 103, 3515). Rajh teaches a method for reducing the band gap energy of TiO2 particles by sensitizing colloidal TiO2 nanoparticles with organic molecules such as dopamine and ascorbic acid. The organic molecules in question have adjacent hydroxide units on an aromatic ring that open the Ti═O double bonds that exist on the surface of the colloidal nanoparticles. While the coupling of colloidal TiO2 with specific organic molecules is very simple, the stability of such bonds is limited. In addition, this coupling relies on the existence of Ti═O bonds. These bonds are associated with “crystal defects” on the surface of the particles, and hence, the density of these bonds is low. Furthermore, the density of these bonds is a function of particle size, which further limits the applicability of this procedure.
Yet another method for modifying the surface of TiO2 particles utilizes sensitizer molecules. For example: Tachibana, et al (J. Phys. Chem. B 2000, 104, 1198) teach a method in which the sensitizer molecules are simply absorbed on the surface of the TiO2 particles. Unfortunately, only a limited number of sensitizer molecules can be so absorbed on the surface of the TiO2 particles, and hence, this technique has a sensitization yield that is insufficient for many applications.